Method and apparatus for resetting multiple processors using a common ROM

ABSTRACT

Two design variations which allow multiple processors to start up using a single ROM. In each design, a single, primary processor is allowed to perform a complete POST while the remaining, secondary processors are directed in the course of their POST to perform a more limited initialization sequence. At power on, the primary processor begins a normal POST, while the secondary processors are held until a vector is placed into a redirection vector location. Each secondary processor is then subsequently started, using its own initialization code located at the address indicated by the redirection vector. The first technique is applicable to general multiprocessor systems because the implementation of this design can be run either from external software or from an addition to the operating system of the particular machine on which it is being used. The second technique is more specifically oriented to a particular system, and includes the use of an identity register to differentiate between primary and secondary processors.

This is a continuation of U.S. application Ser. No. 07/431,743 filed on Nov. 3, 1989, now abandoned.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to multiple processors in computer systems and more particularly to multiple processors having a common reset using a single ROM.

2. Description of the Prior Art

The personal computer industry is a vibrant and growing field that continues to evolve as new innovations occur. The driving force behind this innovation has been the increasing demand for faster and more powerful personal computers. New and increasingly more complex applications for personal computers are continually being developed, and the computer programs required to implement these new applications have experienced a corresponding increase in size and complexity, requiring greater amounts of time for the computer to be able to run them properly. As a result, personal computers have been burdened with lengthier amounts of software instructions that take increasing amounts of time for them to execute.

To meet the challenge of these new software applications, computer designers have used various methods to increase the speed with which personal computers can process instructions. Historically, the personal computer has developed as a system utilizing a single microprocessor to handle all instruction execution. The microprocessor is the key working unit or "brains" of the personal computer, and its task is to handle all of the instructions that programs give it in the form of computer software. The methods that have been used to increase speed in the personal computer have generally centered around maximizing the efficiency with which this single microprocessor can handle instructions.

Limits are being reached, however, on the amount of speed that can be obtained from a system based on a single microprocessor. The obvious choice to remove this constraint has been the incorporation of multiple microprocessors operating in parallel into a computer system. With the use of multiple processors, or multiprocessing, each microprocessor can be working on a different task at the same time. The use of multiprocessing has generally increased computer performance, but it has also resulted in numerous difficulties that were not found in a single processor environment.

A problem that has arisen in multiprocessing has been how to start up or initialize each of the processors without them interfering with each other. In a single processor personal computer environment, the microprocessor generally initializes itself by performing a POST (power on self test). The POST program is located in the computer's ROM, which is a special type of memory that is permanently recorded into the computer. This special memory cannot be written to or changed by software and is nonvolatile, meaning that turning off the computer will not disturb it. The ROM holds a key set of programs that provide essential support for the operation of the computer, among these the test and initialization programs known as the POST, which make sure that the computer is in good working order at power on. The POST program usually includes the microprocessor placing the memory in a known state, testing any memory, placing peripherals in a tested, reset and ready condition and loading or booting up the operating system, among other things.

The problem that arises in multiprocessing is that, in a system where multiple processors coexist sharing a common bus and a common memory, it would be catastrophic for more than one processor to perform a complete POST. For example, if one processor had completed a POST and was up and running, a subsequent processor attempting to perform a POST would re-initialize the peripherals and memory that the previous processor was now using to run its code, resulting in a potential error for the first processor.

For this reason it would seem necessary in a multiprocessing environment for each processor to have its own ROM with which to bring itself to a working state so that one processor's initialization would not interfere with another processor that may already be up and running. However, this scheme would require that numerous ROM's be present in the computer, one associated with each processor, resulting in an unnecessarily large amount of nonvolatile memory being used for this purpose. A further problem that would be associated with the use of multiple ROM's would be the resultant complexity and loading effects on the bus that these ROM's share with the processor. Therefore, it would be highly desirable for these multiple processors to be able to start or "boot" up using a single, common ROM as opposed to each processor containing a different ROM in memory.

For further background on the present invention, it is necessary to examine some of the special features and considerations involving the Intel Corporation (Intel) family of microprocessors, which are used in personal computers compatible with those manufactured and sold by International Business Machines Corp. (IBM). The Intel 80286 microprocessor introduced a new feature that allowed it to operate in two different modes: real mode and protected mode. In real mode, the 80286 behaves very much like the 8088 microprocessor that was in the original IBM PC, thus allowing for full compatibility with these older systems. In protected mode, there are no compatibility considerations, and the 80286 is allowed to utilize all of its special features for maximum capability.

A problem soon became apparent, however, in that, while provisions were made for the 80286 microprocessor to switch from real to protected mode, no provisions were made for the 80286 to return from protected to real mode. This problem was corrected in the Intel 80386 microprocessor, but could only be remedied in the 80286 through the use of the reset operation to return the processor to real mode. The reset operation, however, generally required the computer to perform a complete reset and reboot, and this was found to be unnecessary for the purpose of simply returning the computer from protected to real mode. Therefore, it was determined that some method was needed to indicate whether each reset operation was simply a software reset used for protected to real mode switching or a true system reset, and, as a result, a byte was provided in the CMOS nonvolatile memory available in the computer system to reflect whether or not a full reboot of the system was necessary. At power on, this byte reflects a "normal POST" status, informing the processor that a protected mode reset is not occurring and that a full boot is necessary to begin operation. When the processor is up and running, the status of this byte is changed to reflect a "vector on reset" status when a protected to real mode change is desired, thus informing the processor that a complete reboot is unnecessary.

The processor generally polls the status of this reset byte during its POST program so that the protected to real mode switch can be made with a minimum of lost time. At power on, the normal POST boot status of the reset byte directs the microprocessor to perform a complete POST, whereas, when the processor has been up and running and has its RESET pin toggled, the vector on reset status of the reset byte directs the processor away from the remainder of the POST program to an alternate memory location which is contained in a reset vector location. This memory location is the location desired upon entry into real mode to continue operation of the computer.

SUMMARY OF THE INVENTION

The present invention includes two design variations which allow multiple processors to start up using a single, common ROM. These designs, or techniques, are intended for use in a multiprocessing environment and utilize a system whereby each processor is directed to begin a normal POST, but only a single processor is allowed to perform a complete POST, whereas the remaining processors are directed very early in their POST procedure to perform a limited initialization sequence. The first design is intended for more general applications and is adaptable to any system incorporating multiple processors. This design can be implemented either by the operating system of the particular machine, or by software specifically written for this purpose, with minimal hardware requirements. The second technique is intended to be more particularly adapted to a specific computer and is executed from the POST procedure that is stored in the computer's ROM. This design has a hardware requirement in that an identity register is used to differentiate between the microprocessor performing the complete POST and the remaining microprocessors. This second design also includes a method for allocating tasks to the secondary processors once they are running.

In each of these designs, at power on the processor which performs the complete POST, referred to as the primary processor or processor P₁, begins a POST while the remaining processors which perform alternate initialization sequences, referred to as secondary processors or in the singular as Processor P_(Z), are held in an inactive state. In the first design, an initialization procedure for each processor P_(Z) commences when the processor P₁ has completed its POST program. This initialization procedure includes the processor P₁ placing a vector in the reset vector memory location pointing to initialization code that processor P_(Z) will execute, changing the reset byte in the CMOS non-volatile memory to reflect a vector on reset status, and enabling address line A20. When the processor P_(Z) is activated, it begins a POST as the processor P₁ did, but the vector on reset status in the CMOS reset byte directs the processor P_(Z) to vector off to a different location in ROM containing its initialization code, which it executes. When the processor P_(Z) has been successfully dispatched, it notifies the processor P₁, which then restores the CMOS reset byte and the vector at the reset vector memory location to their original values as well as the original state of A20. This technique is repeated until all the processors have been initialized.

In the second design the initialization process of each processor P_(Z) occurs during the POST program being executed by the processor P₁. A register in the computer's hardware is used to differentiate between the processor that performs the complete POST, the processor P₁, and the remaining processors, represented by the processor P_(Z), which utilize a more limited initialization scheme. Early in the course of the POST sequence, each processor is directed to the identity register, which informs it as to whether it is a primary or secondary processor. If the processor is determined to be the processor P₁, it is allowed to continue with its POST. If the processor is determined to be the processor P_(Z), it is instructed by the POST program to use a vector placed in the reset vector memory location by the processor P₁ to transfer operation to a different location in the ROM containing the more limited initialization code.

Generally, at power on the processor P₁ is activated to begin a POST while the processor P_(Z) remains in a held state. The activation and initialization of the processor P_(Z) is directed from the latter part of the POST procedure that is executed by the processor P₁. The processor P₁ releases the processor P_(Z) and then polls a semaphore location to determine if the processor P_(Z) has completed its initialization. When the processor P_(Z) has completed its initialization, it notifies the processor P₁ and places itself on hold, at which time the processor P₁ completes the initialization of any other processors P_(Z) and then completes the POST.

Upon completion of the POST, the processor P₁ is running, and subsequently the operating system begins allocating various tasks to the processor P_(Z). When the operating system decides to give the processor P_(Z) a task, the processor P₁ takes the processor P_(Z) out of hold and provides it with a vector pointing to the code that the processor P_(Z) is to execute. The processor P₁ and the processor P_(Z) communicate back and forth through a semaphore in memory as to when the processor P_(Z) has begun execution of its task and when it has completed it. When the processor P_(Z) has finished its task, it generally places itself in a held state to make it ready to receive other tasks from the operating system. This cycle of task allocation then repeats itself. The processor P₁ may optionally direct the processor P_(Z) to execute a reset code at any time, this having the utility of resetting the processor P_(Z) to a known state prior to starting an operation.

BRIEF DESCRIPTION OF THE DRAWINGS

A better understanding of the invention can be obtained when the following detailed description of the preferred embodiment is considered in conjunction with the following drawings, in which:

FIG. 1 is a schematic block diagram of a computer system incorporating the present invention;

FIG. 2 is a flow chart diagram of a first design for initializing the multiple processors using a single ROM according to the present invention;

FIG. 3 is a circuit diagram of an identity register called the Who-Am-I port according to the present invention;

FIG. 4 is a flow chart diagram of a second design for initializing the multiple processors using a single ROM according to the present invention;

FIG. 5 is a flow chart diagram of a design for dispatching tasks to secondary processors according to the present invention; and

FIG. 6 is a flow chart diagram of a design for resetting secondary processors after dispatching according to the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

A computer system incorporating the present invention can utilize one of several design variations that allow multiple processors to start up using a single, common ROM with minimal changes to the ROM from single processor ROM's. The following designs will be discussed with specific reference to Intel 80386 or 80486 microprocessors being the microprocessors utilized in the multiprocessor system, but the use of other processors is also contemplated. Throughout the course of this description, the secondary processors will be referred to in the singular as the processor P_(Z), but it is understood that multiple secondary processors may coexist in this environment.

Referring now to FIG. 1, the letter C designates generally a computer system incorporating either of the two designs according to the present invention. Many of the details of a typical computer that are not relevant to the present invention have been omitted for the purpose of clarity. The system C generally includes a primary processor P₁, a secondary processor P_(Z), and various interprocessor logic circuitry 30 that is connected between the primary processor P₁ and the secondary processor P_(Z). Both the primary processor P₁ and the secondary processor P_(Z) each generally include a cache subsystem (not shown). The interprocessor logic 30 generally includes various option and status registers and circuitry relating to the operation of the processors. The primary processor P₁, the secondary processor P_(Z), and the interprocessor logic circuitry 30 are each connected to a system bus 40 which generally includes an address bus and a data bus as well as various control and status lines that allow for the proper functioning of the computer C. Also attached to the system bus is read only memory (ROM) 50, which includes the start-up program that initializes the multiple processors according to the present invention called the power on self test (POST); random access memory (RAM) 52 which forms the main memory of the system C; and CMOS memory 54 which is used to provide nonvolatile, random access memory for use by the system C.

In each of the designs according to the present invention, at power on the primary processor, processor P₁, is activated and begins a POST (power on self test) which is located in the ROM 50 of the computer system, while the secondary processor P_(Z), is kept in a held state. This activation of the processor P₁ and holding of the processor P_(Z) is accomplished in slightly different manners in the two designs. In the first design a reset bit, which is located preferably in a register in the interprocessor logic circuitry 30 referred to as the Processor Option Register is used. The reset bit generally operates similarly for all processors such that a setting of the reset bit, which occurs at power up by hardware control, results in the RESET input of the respective processor being pulsed and causes the respective processor to be placed in a held state. A subsequent clearing of the reset bit releases the respective processor from its held state and allows the respective processor to begin the POST. Therefore, at power on the reset bit of the processor P₁ is toggled, allowing the processor P₁ to begin the POST, while the reset bit of the processor P_(Z) is set but not cleared, thereby keeping it in a deactivated or held state.

The second design utilizes a sleep bit located in the Processor Option Register in the interprocessor logic 30 associated with each processor P_(Z). The sleep bit operates such that, when it is set for a respective processor, requests for the bus 40 by the respective processor are blocked. Therefore, in the second design, the processors P₁ and P_(Z) each have their reset bit toggled and a sleep bit is set on the processor P_(Z). The toggling of the reset bit of the processor P₁ allows it to begin the POST, while the setting of the sleep bit on the processor P_(Z), which occurs at power up by hardware control, causes any requests for the bus by the processor P_(Z) to be blocked, thus effectively placing the processor P_(Z) in a held state. A subsequent clearing of the sleep bit of the processor P_(Z) by the processor P₁ allows bus requests by the processor P_(Z) to be passed, thereby allowing the processor P_(Z) to begin the POST. Therefore, in each of these designs, at power on the reset bit of the processor P₁ is generally toggled by the power on circuitry (not shown) of the computer system C, allowing it to begin a POST, while the processor P_(Z) is kept in a deactivated or held state.

In the first design according to the present invention, as shown in FIG. 2, when the processor P₁ (FIG. 1) has finished a sufficient portion of its POST routine in step 212 to allow start up of the processor P_(Z) (FIG. 1), the software implementing this design performs an initialization procedure I₁ to bring the processor P_(Z) into an active state. First, in step 216, the vector at memory location 40:67 in the ROM 50 (FIG. 1), which is the reset vector memory location, is replaced with a new vector pointing to initialization code located in the ROM 50 that will be executed by the processor P_(Z) when it is enabled. The vector that was previously stored in this location is saved for later restoration.

In step 218, the processor P₁ performs a similar replacement procedure with the CMOS non-volatile memory reset byte, which is preferably located in the CMOS 54. The CMOS reset byte is located at address location OFh in the CMOS memory 54 and is accessed through ports 070h and 071h, as is standard for addressing the CMOS memory in the IBM PC compatible computers. The status of this byte reflects whether a normal boot or vector on reset is necessary when the processor P_(Z) reads this value in the POST sequence. The processor P₁ will have previously interrogated this location and have found the normal boot value present, enabling it to begin a normal POST. The current value of the CMOS reset byte is saved in step 218 to a temporary location, preferably to a register in the processor P₁. The CMOS reset byte is then changed to value 0Ah, this value signifying that the computer system C is or has been running and that only a vector on reset is necessary instead of the normal POST routine that a processor would normally follow at power on. This change, in effect, fools the processor P_(Z) into thinking that it has already performed the POST operations. The 0Ah value written to the CMOS reset byte is the reset without EOI value and is utilized to prevent the Processor P_(Z) from clearing the interrupt controller, as this is unnecessary and might inadvertently cause an error.

Address line A20 of the system address bus 40 (FIG. 1) is enabled in step 220 to allow the processor P_(Z) to properly access high memory in the ROM 50 where the POST program is located. Address line A20 had previously been disabled in the POST routine of the processor P₁ (step 212) for software compatibility reasons, these stemming from the use by previous programmers of a feature of the 8086 microprocessor whereby the program counter rolled over to 0000h after FFFFFh due to the maximum of twenty address lines available in that microprocessor. This roll over was incorporated into software written for these older 8086 microprocessor-based systems for various purposes, the end result for current purposes being that, in order to maintain compatibility with this older software, address line A20 was disabled during the POST of the processor P₁ (step 212). Consequently, address line A20 must be re-enabled in step 220 to allow the processor P_(Z) to address the bootstrap program which is preferably located in high memory in the ROM 50.

In step 222, the processor P₁ clears the reset bit in the Processor Option Register of the Processor P_(Z), thereby activating the processor P_(Z) to begin the POST routine in step 226. The processor P_(Z) comes out of reset and vectors to the reset location in the ROM 50 where the POST program is located. This is the same location where processor P₁ vectored to after reset, this being a general function of the microprocessors used in the present invention. Thus, both the processors P₁ and P_(Z) operate from the same ROM 50 immediately after reset execution. Very early in the execution of the POST routine, the processor P_(Z) polls the CMOS reset byte to determine its status. As the CMOS reset byte was previously changed by the processor P₁ in step 218 to value 0Ah, reflecting a vector on reset status, the processor P_(Z) in step 228 is directed to the reset vector memory location 40:67. This location contains the vector which was placed there earlier by the processor P₁ in step 216, and this vector is used in step 230 to direct the processor P_(Z) to its alternate initialization code, preferably located in the ROM 50 but alternatively located in RAM 52 after being loaded by the processor P₁. This initialization code generally includes the processor P_(Z) executing any specific reset code, testing its cache memory and performing other processor P_(Z) dependent features.

After activating the processor P_(Z) in step 222, the processor P₁ awaits the successful dispatch of the processor P_(Z) in step 224 by polling a semaphore bit which is preferably located in the RAM 52. A final step 232 in the initialization sequence performed by the processor P_(Z) involves the processor P_(Z) altering the semaphore bit to signal to the processor P₁ that the initialization sequence has been successfully completed. When step 232 is completed, the processor P_(Z) begins performing a software loop in step 234, waiting until it is directed by the operating system to perform a task.

When the processor P₁ receives notification by way of the changed semaphore bit that the processor P_(Z) has completed its initialization, the processor P₁ proceeds to step 236 where it restores the original value of the CMOS reset byte from its temporary location. The processor P₁ then proceeds to step 238 where the original vector is returned to the reset vector memory location 40:67 and the initial state of address line A20 is restored. The processor P₁ then continues with its operation.

Referring again to FIG. 1, the second design according to the present invention is similar in many respects to the first, but is tailored for a two processor system C (FIG. 1) incorporating one primary processor, referred to as the processor P₁, and one secondary processor, referred to as the processor P₂. The expansion of this design to incorporate multiple secondary processors, however, is also contemplated.

This second design is also different from the first in that it utilizes an identity register located in the interprocessor logic 30 called the Who-Am-I register to differentiate between primary and secondary processors. The Who-Am-I register resides preferably in the system I/O port space and is used by software to determine whether the processor P₁ or the processor P₂ is currently active. The possible contents of this register include a value 00h to represent that the processor P₁ is currently active, a value F0h to represent that the processor P₂ is currently active, and a value FFh to represent that neither the processor P₁ nor the processor P₂ are active.

Referring now to FIG. 3, the contents of the Who-Am-I register are determined by the hold acknowledge (HLDA) outputs from the processor P₁ and the processor P₂, the HDAKP₁ and HDAKP₂ signals, respectively. The hold acknowledge signal is generally asserted high when the respective processor is in a held or inactive state and is generally negated low when the respective processor is currently active. In the preferred embodiment if a processor P₁ or P₂ is reading the location it is active and the other processor must be inactive to allow the active processor access to the register, thus allowing the determination. The two hold acknowledge signals HDAKP2 and HDAKP₁ are inverted by inverters 106 and 104, respectively. The output of the inverter 104 and the HDAKP₂ signal are inputs to a two input AND gate 108, whose output is the P1ON signal. The P1ON signal is connected to the input of an inverter 116, whose output provides the upper four bits of the Who-Am-I register. The output of the inverter 106 and the HDAKP₁ signal are inputs to a two input AND gate 110, whose output is the P2ON signal. The P1ON and P2ON signals are inputs to a two input NOR gate 118, whose output provides the lower four bits of the Who-Am-I register. Therefore, by polling the status of the Who-Am-I register, the processor is able to determine whether it is the processor P₁ or the processor P₂.

The Who-Am-I register can be readily expanded to indicate the active status of a greater number of processors than the two utilized in the present invention. This expandability can be achieved by reducing the numbers of output bits driven for each processor and incorporating the respective hold acknowledge signals of each of the additional processors through the appropriate logic, allowing for up to 8 processors to be identified in each byte.

Referring now to FIG. 4, the second design begins similarly to the first in that in step 252 the processor P₁ is allowed to begin a POST, while in step 254 the processor P₂ is held inactive. At power on the processor P₁ generally has its reset bit toggled, allowing it to begin a POST. The processor P₂ also has its reset bit toggled, but the sleep bit of the processor P₂ is set, and this has the utility of blocking any bus requests by the processor P₂, thereby keeping the processor in a held state. Early in the execution of the POST sequence in step 252, the processor P₁ is directed to the Who-Am-I register (FIG. 3), which informs it that it is in fact the processor P₁, thereby allowing it to resume execution of the POST program. During the remaining course of the POST, the processor P₁ is directed by the POST program to execute the following initialization procedure for the processor P₂.

In step 256 the processor P₁ initializes a processor available bit register, preferably located in the interprocessor logic 30 (FIG. 1) of the computer C, from configuration information contained in the CMOS memory 54. The processor available register contains a sequence of bits that serve as an indicator to the various processors as to what other processors are available to be given tasks to perform. In general, each secondary processor preferably has a representative availability bit in the processor available register that reflects whether or not that processor is available for dispatching. If the respective availability bit of a secondary processor is set, then that processor is available for dispatching, and if the respective availability bit of a secondary processor is cleared, then that processor is not available for dispatching and is assumed to be either currently dispatched or not present within the system. If the CMOS memory 54 is found to be invalid during initialization, all of the availability bits in the processor available register are cleared. The processor available register is initialized in step 256 to reflect the present configuration of the system, this being which secondary processors, represented by the processor P₂, coexist in this environment with the primary processor P₁. The processor available register in the second design according to the present invention is configured to include two microprocessors, one primary processor P₁ and one secondary processor P₂, but a configuration of the processor available register to incorporate a greater number of microprocessors than two is also contemplated.

In step 258, the processor P₁ proceeds with the initialization procedure by storing the current vector from the reset vector memory location 40:67 into a temporary location and replacing it with a new vector pointing to initialization code that the processor P₂ will execute when it is enabled. In step 260, the processor P₁ activates the processor P₂ by clearing the sleep bit in its Processor Option Register, enabling the processor P₂ to begin the POST program in step 264. After enabling the processor P₂ in step 260, the processor P₁ begins polling the sleep bit associated with the processor P₂. The sleep bit is used here as a handshake between the processor P₁ and the processor P₂ to indicate when the processor P₂ has finished its initialization procedure and has been placed on hold. Preferably, if processor P₁ does not see the sleep bit set by the processor P₂ after a certain period of time, the processor P₁ sets the reset bit of the processor P₂, effectively placing the processor P₂ in a held state, and then continues with the POST. A different Processor Option Register is preferably used for each additional secondary processor incorporated into the multiprocessing environment.

The processor P₂ begins the POST sequence in Step 264 after the processor P₁ clears its sleep bit. The processor P₂ is directed in step 266 to the Who-Am-I register (FIG. 3) to determine its identity, just as the processor P₁ was in step 252 during its execution of the POST program. The Who-Am-I register informs the processor P₂ that it is a secondary processor and directs it to vector off based on the value in the reset vector memory location 40:67. This memory location contains the vector previously placed there by the processor P₁ in step 258, and this vector points to the alternate initialization code which the processor P₂ executes in step 268. The alternate initialization code is preferably located in the ROM 50 but may be alternatively located in RAM 52 after being loaded by the processor P₁. This initialization code generally includes the processor P₂ executing any specific reset code, testing its cache memory, and programming its noncacheable address map, as well as any other processor P₂ dependent features. Upon completion of its initialization code in step 268, the processor P₂ places itself on hold in step 270 by setting the sleep bit in its own Processor Option Register. The setting of the sleep bit places the processor P₂ in a hold state as soon as the bus 40 must be requested and serves as notification to the processor P₁ that the processor P₂ has finished its initialization procedure. Completing step 270, the processor P₂ performs a jump or branch instruction requiring that the value at the reset vector 40:67 be obtained. Because this results in a bus request, the processor P₂ goes into a held state. When the processor P₁ receives this notification in step 272, it resumes execution of the remainder of its POST.

Upon completion of the POST, the primary processor, processor P₁, is running, and the operating system subsequently begins an allocation of various tasks to each secondary processor. The following allocation procedure will be discussed with reference to a computer system C incorporating a particular secondary processor referred to as the processor P₂, but the incorporation of multiple secondary processors in the task allocation scheme is also contemplated. Referring now to FIG. 5, when the operating system has a task for the processor P₂ to perform, it directs the processor P₁ to perform the following dispatch procedure for the processor P₂.

The processor P₁ first checks the status of the respective availability bit of the processor P₂ in the processor available register in the interprocessor logic 30 in step 282 to determine if the processor P₂ is available for dispatching. This availability bit was initially set by the processor P₁ in step 256 (FIG. 4) according to the configuration information held in the CMOS memory 54 and this bit is subsequently cleared whenever the processor P₂ is given a task to perform. If the availability bit of the processor P₂ is not set, then the operating system knows that either the processor P₂ has not yet finished the task that it was previously given or that the processor P₂ is not presently configured to the system. In either case, the processor P₂ is determined to be unavailable. If the availability bit of the processor P₂ is set, then the operating system commences with the allocation of the task in step 284 by placing a vector in memory location 40:67 pointing to software which generally includes the task that the operating system wishes the processor P₂ to execute. The processor P₁ saves the previous value from memory location 40:67 for later restoration.

The processor P₁ then activates the processor P₂ in step 286 by clearing the sleep bit in the processor P₂ 's Processor Option Register located in the interprocessor logic 30 in step 286, causing the processor P₂ in step 290 to obtain the vector at memory location 40:67 to begin operation of the new task. An attempt to obtain the vector at memory location 40:67 was actually the last instruction executed by the processor P₂ in the POST procedure in step 270. This vector fetch is by definition a cache miss so that the processor P₂ must use the bus. However, the sleep bit was set in step 270, disabling any bus request, so the vector fetch is pending, waiting for the sleep bit to be cleared. Once the sleep bit is cleared, the processor P₂ can properly access the bus and obtain the vector at memory location 40:67 pointing to the task that it is to execute. The first instruction in this new task is step 292 which directs the processor P₂ to clear its respective availability bit in the available bit register to indicate to the operating system that it is no longer available for dispatching and that it has commenced with the task. After this, the processor P₂ begins executing the task in step 294.

The clearing of the availability bit by the processor P₂ serves as notification to the processor P₁ that the processor P₂ has begun execution of the task that the operating system has given it. Upon receiving this notification, in step 296 the processor P₁ restores the previous vector to the reset vector memory location 40:67 and then resumes the execution of its own code. When the processor P₂ has finished executing its task in step 294, it sets the sleep bit in its Processor Option Register in step 298, blocking any further bus requests, and then attempts to obtain the reset vector at memory location 40:67. In this way, operation of the processor P₂ is again held at the vector fetch operation as described previously in step 270, and processor P₂ is available to receive a new task from the operating system.

In certain cases, it may be necessary to reset the processor P₂ to a known state in order to make it available for further dispatching. In these cases, the processor P₁ performs the following Reset-After-Dispatch routine to reset the processor P₂ to a known state. Referring now to FIG. 6, in step 312, the processor P₁ first places a vector in memory location 40:67 pointing to the reset code that the processor P₂ is to execute, saving the previous value in this memory location for later restoration. The processor P₁ then activates the processor P₂ from its hold or operating state in step 310 by toggling the processor P₂ 's reset bit in step 314, forcing the processor P₂ to reset and request the system reset memory location, which is the beginning of the POST program. The POST sequence is started in step 318. Early in the course of the POST, the processor P₂ is directed in step 320 to the Who-Am-I register (FIG. 3), which informs it that it is a secondary processor and directs it to the reset vector memory location 40:67. This location contains the vector placed there previously by the processor P₁ in step 312 pointing to the reset code, which the processor P₂ executes in step 322. This reset code is generally similar to the reset code that processor P₂ executes during its initialization in step 268 (FIG. 4) at power on and includes the setting of the processor to a known state. Note, however, that the reset code that the processor P₂ executes in step 322 may not include the other procedures that the processor P₂ performed during its original initialization such as testing the cache and programming the noncacheable address map. When the processor P₂ has finished executing its reset code in step 322, it notifies the processor P₁ by setting the sleep bit in its Processor Option Register in step 324. The processor P₂ also initiates a request to transfer control to the vector contained at memory address 40:67 in step 324, but since its sleep bit is set, all bus accesses by the processor P₂ are blocked, and it is therefore essentially in a held state waiting for its sleep bit to be cleared. In this way, the processor P₂ is available for further dispatching in the manner described above in that the processor P₂ obtains directions to its new task from memory location 40:67 when its sleep bit is cleared.

After resetting the processor P₂ in step 314, the processor P₁ proceeds to poll the processor P₂ 's sleep bit in step 316, waiting for the Processor P₂ to indicate that the reset operation has been completed. When the processor P₁ sees that the sleep bit of the processor P₂ is set, it proceeds to step 326 where the processor P₁ sets the availability bit of the processor P₂ in the processor available register. This signifies that the processor P₂ is now available for dispatching. The processor P₁ then returns to the execution of its code in step 328. When the operating system has another task for the processor P₂ to perform, it repeats the allocation cycle of FIG. 5 and, in some instances, the Reset-After-Dispatch routine of FIG. 6, as necessary.

The foregoing disclosure and description of the invention are illustrative and explanatory thereof, and various changes in the procedures, components, and circuit elements, as well as in the details of the illustrated circuitry and method of operation may be made without departing from the spirit of the invention. 

What is claimed is:
 1. A method for powering up multiple processors in a computer system that includes a primary processor which commences operation on power-up of said computer system, a secondary processor which enters a hold state on power-up of said computer system, the primary processor and the secondary processor being powered up together, common elements used by both said primary processor and secondary processor, means coupled to said primary processor and said secondary processor for allowing said hold state of said secondary processor to be terminated, means for storing a redirection vector, and means for storing a start-up program which includes as one step determining whether said redirection vector is to be utilized, said start-up program including initialization code used to initialize said secondary processor, the method comprising:said primary processor commencing said start-up program and not utilizing said stored redirection vector, said start-up program including steps for initializing said primary processor and steps for initializing said common elements of the computer system; said primary processor completing appropriate initialization of said computer system; said primary processor placing a redirection vector pointing to said secondary processor initialization code in said redirection vector storing means; said primary processor activating said secondary processor; said secondary processor commencing said start-up program and utilizing said stored redirection vector to begin execution of said secondary processor initiation code after said primary processor places said redirection vector, wherein said secondary processor initialization code is only a subset of operations comprised in said start-up program and does not include said steps for initializing said common elements of said computer system; and said secondary processor executing said secondary processor initialization code.
 2. The method of claim 1, wherein said computer system further includes a read only memory coupled to both said primary processor and said secondary processor for storing said start-up program and said initialization code.
 3. The method of claim 1, wherein said computer system further includes means for storing information indicating whether said redirection vector should be utilized and wherein said start up program references said storing means to determine redirection vector utilization, the method further comprising:said primary processor placing information in memory indicating the utilization of said redirection vector before activating said secondary processor.
 4. The method of claim 1, wherein said computer system includes only one said primary processor and only one said secondary processor.
 5. The method of claim 1, wherein said computer system includes a means for differentiating between said primary processor and said secondary processor and said start up program references said means for differentiating to determine redirection vector utilization.
 6. The method of claim 3, wherein said redirection vector utilization information storing means includes CMOS nonvolatile memory.
 7. The method of claim 3, wherein said computer system further includes means for storing information indicating that said secondary processor has completed said initialization code, the method further comprising:said primary processor polling said initialization complete storing means after activating said secondary processor; said secondary processor placing information in said initialization complete storing means indicating that said initialization code has been executed when said initialization code has been completed; and said primary processor resuming operation after said secondary processor has placed information in said initialization complete storing means indicating that said initialization code has been completed.
 8. The method of claim 3, wherein said computer system further includes means for placing said secondary processor on hold and removing said secondary processor from a hold state, the method further comprising:said secondary processor placing itself on hold after executing said initialization code.
 9. The method of claim 7, wherein said computer system further includes means for temporarily storing the original contents of said redirection vector storing means and said redirection vector utilization information storing means, the method further comprising:said primary processor placing original contents of said redirection vector storing means into said temporary storing means before placing said redirection vector into said redirection vector storing means; said primary processor placing original contents of said utilization information storing means into said temporary storing means before placing information into said utilization storing means indicating the utilization of said redirection vector; and said primary processor restoring said original contents of said redirection vector storing means and said utilization information storing means from said temporary means after said secondary processor places information in said initialization complete storing means indicating that said initialization code has been executed.
 10. The method of claim 9, wherein said computer system further includes means for storing information indicating that said secondary processor has completed said initialization code, the method further comprising:said primary processor halting operation and polling said initialization complete storing means after activating said secondary processor; said secondary processor placing information in said initialization complete storing means indicating that said initialization code has been executed when said initialization code has been completed; and said primary processor resuming operation after said secondary processor has placed information in said initialization complete storing means indicating that said initialization code has been completed.
 11. The method of claim 9, wherein said computer system includes a means for placing said secondary processor on hold and removing said secondary processor from a hold state, the method further comprising:said secondary processor placing itself on hold after executing said initialization code.
 12. The method of claim 9, where said computer system includes only one said primary processor and only one said secondary processor.
 13. A computer system having multiple processors configured in a multiprocessor environment sharing memory and input/output space, said computer system comprising:a primary processor which commences operation on power-up of said computer system; a secondary processor which enters a hold state on power-up of said computer system, said secondary processor powering-up at the same time as said primary processor; means coupled to said primary processor and said secondary processor for allowing said hold state of said secondary processor to be terminated; means coupled to said primary processor and said secondary processor for storing a start-up program which determines whether a redirection vector is to be utilized, which is begun by both said primary processor and said secondary processor upon activation of operation, said start-up program including steps for initializing said primary processor and steps for initializing common hardware elements usable by both said primary processor and secondary processor, said start-up program further including initialization code for said secondary processor, said secondary processor initialization code being comprised of a subset of operations comprising said start-up program and not including said steps for initializing said common hardware elements; means for storing a redirection vector pointing to said secondary processor initialization code; and means for directing said secondary processor to utilize said redirection vector to execute said secondary processor initialization code.
 14. The computer system of claim 13, wherein said start up program storing means includes a read only memory.
 15. The computer system of claim 13, further including means coupled to said primary processor and said secondary processor for allowing communication between said primary processor and said secondary processor.
 16. The computer system of claim 13, incorporating only one said primary and only one said secondary processor.
 17. The computer system of claim 13, wherein said means for directing said secondary processor to utilize said redirection vector storing means includes a CMOS memory.
 18. The computer system of claim 13, wherein said means for directing said secondary processor to utilize said redirection vector storing means includes a means for differentiating between said primary processor and said secondary processor.
 19. The computer system of claim 13, further comprising means for storing information indicating that said secondary processor has completed said initialization code, said storing means being polled by said primary processor while said secondary processor is executing said initialization code.
 20. The computer system of claim 13, further comprising means for temporarily storing original contents of said redirection vector storing means and said redirection vector utilization information storing means.
 21. The computer system of claim 13, further comprising means for placing said secondary processor on hold and removing said secondary processor from a hold state. 